Re: Optimizing execution of expensive subqueries

Mathieu Fenniak <mathieu.fenniak@xxxxxxxxxxxx> · Fri, 13 Jul 2018 07:34:39 -0600

Hi Hellmuth,
Thanks for the response and the new approach; a LATERAL JOIN is new to me.  Unfortunately it seems to have the same performance characteristics and query plan.  The aggregation in the lateral join still executes for every row (eg. if my base query has 500000 rows, I get "Aggregate (... loops=500000)" in the query plan), unaffected by the later LIMIT node in the query plan.

The CTE approach seems to be the only one I can use to improve performance right now, but requires significant application code changes.

Mathieu

On Wed, Jul 11, 2018 at 1:55 PM Hellmuth Vargas <hivs77@xxxxxxxxx> wrote:
Hi
Try this way:

SELECT
  tbl.field1, tbl.field2, tbl.field3, ...,
  b.Thingy1Sum,
  ... repeat for multiply thingies ...
FROM
  tbl
  LATERAL JOIN (
	SELECT anothertbl.UserId,SUM(Duration) as Thingy1Sum
	FROM anothertbl 
	WHERE anothertbl.UserId = tbl.UserId AND anothertbl.ThingyId = 1
	group  by 1) as b on tbl.UserId=b.UserId
ORDER BY tbl.field1 LIMIT 20

El mié., 11 de jul. de 2018 a la(s) 09:25, Mathieu Fenniak (mathieu.fenniak@xxxxxxxxxxxx) escribió:
Hi pgsql-general!
I'm currently looking at a query that is generally selecting a bunch of simple columns from a table, and also performing some subqueries to aggregate related data, and then sorting by one of the simple columns and paginating the result.

eg. 

SELECT
  tbl.field1, tbl.field2, tbl.field3, ...,
  (SELECT SUM(Duration) FROM anothertbl WHERE anothertbl.UserId = tbl.UserId AND anothertbl.ThingyId = 1) as Thingy1Sum,
  ... repeat for multiply thingies ...
FROM
  tbl
ORDER BY tbl.field1 LIMIT 20

I'm finding that if "tbl" contains hundreds of thousands of rows, the subqueries are being executed hundreds of thousands of times.  Because of the sorting and pagination, this is appears to be unnecessary, and the result is slow performance.  (PostgreSQL 9.5.9 server)

I've only found one solution so far, which is to perform the sort & pagination in a CTE, and the subqueries externally.  Are there any other approaches that can be taken to optimize this and prevent the unnecessary computation?

CTE rewrite:

WITH cte AS (
SELECT
  tbl.field1, tbl.field2, tbl.field3
FROM
  tbl
ORDER BY tbl.field1 LIMIT 20
)
SELECT cte.*,
  (SELECT SUM(Duration) FROM anothertbl WHERE anothertbl.UserId = tbl.UserId AND anothertbl.ThingyId = 1) as Thingy1Sum,
  ... repeat for multiply thingies ...
FROM cte;

Thanks for any thoughts you have,

Mathieu Fenniak

-- 
Cordialmente, 

Ing. Hellmuth I. Vargas S. 